Multi-pass laser resonators, also called multi-fold or folded laser resonators, are used in lasers to achieve a long effective gain path while maintaining a short physical length for the resonator cavity. A multi-pass laser resonator, also referred to as a multi-pass optical cavity, may be formed by folding a stable single pass resonator one or more times via one or more mirrors. Although the folding of the beam path can occur in one, two, or three dimensions, linearly folded multi-pass resonators have the advantage of being relatively easy to construct. Within the linear multi-pass configuration, complete free space, complete waveguide or hybrid operation can define the axes transverse to the beam path. While the use of a waveguide in the folded axis can constrain laser oscillation to a single mode, it is sometimes desirable to use free space propagation instead, due to the ability to more easily achieve a Gaussian beam in free space.
The fundamental mode of a laser resonator may be characterized by a beam whose beam shape follows a Gaussian intensity distribution. As used herein, the radius of a Gaussian laser beam is defined to be the distance (from the center location of peak intensity in the beam) at which the intensity of the beam is reduced by a factor 1/e2. Furthermore, as used herein the waist position of a Gaussian beam is defined to be the longitudinal position along the beam where the smallest radius exists.
Multi-pass laser resonators that operate in a hybrid configuration have output beams that have highly asymmetric cross-sections, also referred to as spot sizes, and/or are highly astigmatic (i.e., the beam waist in one dimension is located at a different axial position along the beam than a beam waist in a perpendicular direction.) Because of this problem, many multi-pass slab lasers require expensive external beam shaping optics to remove the asymmetry and astigmatism in the output beam in order for them to be useful for practical applications such as laser cutting, marking, etching, or the like.